1. Field of the Invention
This invention relates in general to a structure and method of manufacturing read only memories (ROMs), and more particularly to a structure and method of manufacturing a ROM with diode memory units located in separate parallel trench-like bit lines that can be programmed to either an ON state or an OFF state through processing and having the additional benefit of an increased ROM density.
2. Description of the Related Art
ROMs are now widely used in digital equipment such as microcomputers and microprocessor operating systems. Resident programs used by the operating systems, such as BIOS, (Basic Input/Output System) are normally kept in ROMs. Due to rather complicated manufacturing processes for ROMs involving a lot of time performing steps and material processing, generally customers will submit their program codes to a memory manufacturer, and then the manufacturer generates masks accordingly, before coding the information into semi-finished memory taken from warehouse stock.
To a certain extent, the designs of modern memories are limited by the advances in semiconductor manufacturing technologies, and therefore any further reduction of the feature size of memory units requires continued improvements in the repertoire of manufacturing techniques. Conventional ROMs are composed of a matrix of metallic oxide semiconductor field effect transistors (MOSFETs) with each field effect transistor representing one memory cell unit. Each field effect transistor so formed provides one of two characteristic selected values for the transistor. For example, one such characteristic is the threshold voltage of the transistor. By implanting impurities into the channel region of a transistor, the transistor will have a lower threshold voltage and can be switched open more readily when a gate voltage Vcc is applied. Without any impurities implanted in the channel region of a transistor, the transistor will have a higher threshold voltage and therefore will not be turned on by the application of the same gate voltage Vcc. Through the selective implantation of impurities into the channel regions of transistors, binary data bits are stored in the memories. Those transistors having a channel implantation can represent the storage of a logic "0", while those transistors having no channel implantation can represent the storage of a logic "1".
FIG. 1A is a top view of part of a conventional ROM. The ROM consists of a plurality of separate parallel bit lines 12a.about.12c embedded beneath field oxide areas 200 and acting as the source/drain diffusion regions for the transistors. These bit lines, together with the different impurity concentrations doped between the lines, determine whether data bits "0" or "1" are stored in the resulting memory units. Referring to FIG. 1A, memory transistor units 16a and 16d are in an ON state while units 16b and 16c are in an OFF state. Overlying and running across the bit lines are a plurality of parallel word lines such as word lines 18 (WL1) and 18' (WL2), through which the stored data are accessed by the application of a voltage. FIG. 1B is a perspective view of part of the cut-out section along the section line II--II of FIG. 1A. The section consists of a P-type semiconductor substrate 10, embedded bit lines 12a, 12b and 12c beneath the field oxide areas 200, a thin insulating area 14 and word lines 18 and 18' crossing over the bit lines. FIG. 1C is an equivalent circuit diagram corresponding to the ROM unit shown in FIG. 1B.
The ROM specified above uses a channel transistor as its fundamental memory unit. Therefore, a rather large volume is occupied by each individual component, and in addition, in the process of miniaturizing component dimensions, the short channel effect emerges, resulting in a rather large leakage current. Besides, the operating current of the memory is limited by the threshold voltage (Vt) of the transistor, and hence when the word line is operating at a low voltage, the starting current of the memory transistor may be too weak to differentiate separate data bits in read/write operations.